According to Louise Gardiner:
"In the foreground Herakleitos (probably a portrait of Michelangelo) broods
alone. Diogenes sprawls on the steps. At right students surround Euclid, who
demonstrates a theorem. This group is especially interesting; Euclid may a
portrait of the aged Bramante, and at the sxtreme right Raphael places is own
I also believe that Plato, in the center is a portrait of Leonardo.